Calcination

Calcination

Calcinations is the process of subjecting a substance to the action of heat, but without fusion, for the purpose of causing some change in its physical or chemical constitution. The objects of calcination are usually: (1) to drive off water, present as absorbed moisture, as "water of crystallization," or as "water of constitution"; (2) to drive off carbon dioxide, sulphl11' dioxide, or other volatile constituent; (3) to oxidize a part or the whole of the substance. 'there are a few other purposes for which calcination is employed in special cases, and these will be mentioned in their propel' places. The process is often called "roasting," "firing," or "burning," by the workmen. It is carried on in furnaces, retorts, or kilns, and very often the material is raked over or stirred, during the process, to secure uniformity in the product. The furnaces used for calcining substances vary much in their construction, but there are three general classes: muffle, reverberatory, and shaft furnaces or kilns.

Muffle furnaces (Fig. 10) are so constructed that neither the fuelnor the fire gases come in direct contact with the material to be calcined. A retort (A) of iron, brickwork, or fire-clay, is placed over the fire grate (G). Flues (F. F) are built around the retort, and through these the hot gases from the fire pass on their way tothe chimney (E).Reverberatory furnaces are built in many forms, but in all cases the flames and hot gases from the fire come in direct contact with the material to be calcined, but the fuel is separated from it. The simplest and most common form is shown in Fig. 11. The fire burns on the grate at (G), and the flames, passing over the bridge at (E), are deflected down ward by the low sloping roof of the furnace, and pass directly over the surface of the charge in the bed of the furnace at (B), finally escaping through the throat (F) into the chimney. The charge is spread out in a thin layer on the bed (B), and may be either oxidized or reduced according to the method of firing and the amount of air admitted. The revolving furnace (Figs. 3 and 39) is a very important modification of the reverberatory furnace. This consists of a horizontal or slightly inclined cylinder (B) of iron or steel plates, lined with fire-brick or other suitable fire-resisting material, and open at each end. The flames from a grate (A) at one end pass through it on their way to the chimney (D). The cylinder is revolved about its. longitudinal axis by means of a gear. It is turned until a manhole in the side is brought directly under a hole in the floor above, the bolted cover is removed, and the charge dumped in. The revolution of the cylinder stirs the charge thoroughly, and brings it into intimate contact with the flame. To discharge the contents, the cylinder is stopped when the manhole is on the under side, the cover is removed, and the material drops out upon the floor or into a car placed for it. To facilitate discharging, the lining usually slopes from all sides towards the manhole. The speed varies from about two revolutions a minute to one revolution in five or ten minutes. These furnaces are now extensively used, their advantages being the intimate mixing and even heating of the charge, and the large quantities, amounting often to several tons, which can be worked at one time. Shaft furnaces and kilns are of two general classes, periodic and continuous. After a charge has been calcined, the periodic furnace (p. 149) or kiln is allowed to cool before it is emptied and recharged. In the continuous variety (p. 148) this is not necessary, and the calcined substance is withdrawn and fresh material added without loss of time or waste of heat. The furnaces may be charged with alternate layers of fuel and material to be calcined. By this method, known as "burning with short flame," the material to be calcined is in close contact with the fuel, and is of course more or less contaminated with ashes. In other forms of shaft furnaces (Fig. 56) the fuel is burned on a separate grate, and only the flames and hot gases pass into the shaft; consequently, no ashes are left in the product. This process is called" burning with long flame." Any of the various forms of furnace here mentioned may be heated by natural gas, generator gas, or oil. This is very advantageous in the matter of cleanliness and of regularity of temperature.(See Fuels.)